1. Field of the Invention
The invention relates to the activation or preconditioning, or both, of porous gas purification substrates. More particularly it relates to activation or preconditioning of such substrates rapidly and with reduced gas usage.
2. Background Information
Purification (decontamination) of many types of gases is done by passing a contaminated gas or gas mixture over a porous substrate within a vessel. The contaminants in the gas are trapped on active sites on the surface of the substrate and the gas upon exiting from the vessel has a much reduced concentration of contaminants. This type of gas purification is commonly used to reduce the contaminant concentration of a manufacturing process gas which is subsequently to be used in the manufacture of high purity materials such as semiconductor wafer materials and prosthetic devices. In such purification processes the contaminate level of the gas is often reduced to or below 1 ppm and in many processes can be reduced into the range of parts per billion (ppb) and even in some cases into the parts per trillion (ppt) range.
Substrates may be in the form of flat or curved plates, small shaped objects such as rings, spheres, saddles or the like, or be particulate or granular materials. While the actual substrate form is often a matter of choice based on factors such as acceptable gas pressure drop through the vessel, the substrate normally must be highly porous so that it has a high surface area, since contaminate removal from gases is essentially a surface phenomenon. Many porous substrates have surface areas on the order of 100 square meters per gram (m2/g) or higher.
When a new substrate material is initially placed into a gas purification vessel the pores of the substrate are filled with a packing gas, which is simply environmental gas to which the substrate has previously been exposed. Commonly this is air or an inert gas. Since the presence of the packing gas in the pores blocks access of the contaminated gas to many of the active removal sites on the surface of the substrate, the packing gas must be removed by purging and the substrate saturated, usually with the same gas as will be purified or a component thereof, before the purification operation can begin. The same type of initial gas removal must also be performed when a vessel has been shut down and vented (such as for repairs) and is to be again placed in operation. This removal and replacement process is commonly referred to as xe2x80x9cpreconditioningxe2x80x9d of the substrate.
In some preconditioning processes chemical reactions may also occur, generating water vapor or other gaseous by-products. The flow of the preconditioning gas must also continue until the reactions have ceased and the gaseous by-products have been purged from the system.
There is an equivalent process used when the active sites on a substrate are of only limited decontamination activity initially. Such sites must be xe2x80x9cactivatedxe2x80x9d by contacting them with an activating gas, causing them to become much more active for decontamination. The mechanism of activation is not important for this invention. What is important, however, is that the activating gas must come into contact with the surface sites of the substrate in order to activate them. Thus the purging gas must be forced to as many of the activation sites as possible during activation. A particular substrate may require both activation and preconditioning, which may occur simultaneously or in sequence, and may be accomplished either by different gases or by the same gas.
It will be evident that for both preconditioning or activation processes it is important that the packing gas be removed from all areas of the surface of the substrate and that all sites must be contacted if they are to activated. While this is readily accomplished for those surface sites and areas to which easy access of a flowing preconditioning or activation gas can be obtained, such as the outer surface of the substrate plate, object or granule, it becomes much more difficult for those areas of the substrate that are deep within the pores of the substrate.
In past gas purification processes, activation and preconditioning gases have been flowed through the vessel and across the substrate and have reached into the pores of the substrate by mass transfer/molecular diffusion. Very long activation or preconditioning periods have been required since such diffusion occurs slowly, particularly as the gas traverses into greater depths of the pores. It is quite common for it to require 24 to 48 hours for satisfactory activation or preconditioning of an entire substrate to be accomplished by flow-generated mass transfer/molecular diffusion. In addition, such diffusion does not provide thorough activation or preconditioning, since as a pore narrows over its length, there is greater resistance to diffusion of the purging or activating gas through it, such that many sites requiring activation or areas requiring purging of packing gases simply cannot be reached by the slowly diffusing gas within a reasonable period of time. During prolonged preconditioning or activation periods required, it is not uncommon to have excessive exotherms occur within the substrate. In order to avoid such exotherms (which could damage the substrates) it is often necessary to limit the flow rate of the purging gas through the vessel, thus also reducing the rate of diffusion of the purging gas into the pores and prolonging the activation or preconditioning time period.
Forced convection purging of equipment has been used in some of the chemical and petroleum industries, but it has been with respect to macro-scale processes in which only relatively coarse and limited removal of packing gases or, limited activation of active sites has been required. Such has not previously been known in or believed applicable to gas purification reactors and vessels in which ultra-high purity (xe2x89xa61 ppm contamination) must be accomplished.
The present invention overcomes the problems of activation and preconditioning in prior art gas purification systems, and materially speeds up activation and preconditioning, generally eliminates the likelihood of excessive exotherms, and permits much more thorough saturation of the substrate with the activation and preconditioning gases. This is accomplished in the present invention by providing forced convection of the preconditioning or activating gases through the pores of the substrate. (For brevity herein, preconditioning and/or activation gases will sometimes be referred collectively or singly as xe2x80x9cpurgingxe2x80x9d gases.) In the process of the present invention a purging gas or gas mixture to be used for preconditioning or activating is pumped into the substrate-containing vessel, on which all outlet ports have been closed. The amount of gas used is sufficient to raise the gas pressure to several times the xe2x80x9catmosphericxe2x80x9d pressure. The elevated pressure is maintained for a short predetermined time and then the outlet ports of the vessel are opened and the contents of the vessel are vented to atmospheric pressure. This will include both purging gas and packing gas displaced during the pressurization. Promptly thereafter the outlet ports are closed and the vessel is again pressurized with the purging gas to an elevated level, and the elevated pressure is maintained for a short predetermined time, again followed by venting of the vessel. This cycle is repeated as often as needed or desired.
We have determined that such pressurizing and venting cycles when repeated for at least two, preferably at least four, and more preferably at least ten, times will result in most if not all of the sites being activated and most if not all of packing gas (and any gaseous byproduct if a chemical reaction has occurred) being purged from the substrate and vessel. The forced convection of this process causes the purging gas to be forced through essentially all of the narrowest portions of the porous substrate, such that virtually all activation sites and gas-containing recesses are reached by the purging gas, in a manner much more rapid and much more thorough than what is accomplished by diffusion of a purging diffusion. This results in not only much more rapid and complete activation or preconditioning, but also in the use of far less purging gas than is required for the long flow periods necessary for diffusion activation or preconditioning.